This proposal is concerned with the biological function of the actin modulatory protein gelsolin during apoptosis, cell transformation, and malignancy. Gelsolin is best known for its ability to effect dynamic changes in the actin cytoskeleton during many forms of motility. However, gelsolin has recently been found to be a substrate for the apoptosis enzyme caspase-3, and may serve a dual role as both a regulator or brake of apoptosis, and an effector of the morphologic changes of apoptosis. In addition, an increasing number of studies have shown that gelsolin expression is dramatically down-regulated during malignant transformation in multiple organ systems. Moreover, gelsolin expression in early stage non-small cell lung cancer appears to provide highly significant negative prognostic information in patients with those tumors. Therefore, gelsolin appears to be a critical protein during carcinogenesis, whose expression is shut off during transformation and later upregulated as metastatic potential increases. The specific role of gelsolin in apoptosis and malignancy will be explored in the following specific aims. First, the role of gelsolin during apoptosis will be elucidated. Multiple primary cell cultures from wild type and gelsolin null mice will be examined under different inducing stimuli to see if gelsolin expression positively or negatively regulates apoptosis, as seen with neutrophils and murine embryo fibroblasts cells respectively. Co-treatments that enhance inhibition of apoptosis by gelsolin will also be sought. Second, the role of gelsolin during malignant transformation will be examined using in vivo murine models. The gelsolin gene defect will be bred into mice engineered or treated to have tumor predisposition: p53 null mice, Tsc2 null mice, mice treated with ENU to induce bladder tumors, and mice treated with urothane to induce lung tumors. Third, the role of gelsolin in lung carcinogenesis will be examined using lung cancer cell lines. The mechanism of down-regulation of gelsolin expression will be explored. Gelsolin will be transfected into cultured human lung cancer cell lines, and their in vitro growth, motility, and phosphoinositide metabolism will be examined, as well as their in vivo growth and metastatic potential which will be evaluated after orthotopic transplantation in nude mice. The results should provide major insight into the function of gelsolin in these processes and provide potential therapeutic strategies to modulate apoptosis and malignant potential.